1. Field of the Invention
The present invention relates to a wireless communication system using an impulse signal train, a wireless transmission apparatus, a wireless reception apparatus, a wireless transmission method, and a wireless reception method.
2. Description of the Prior Art
Owing to the recent information communication technology, many offices and homes are introducing LANs. There are increasing demands of wireless LANs which do not require the wiring for the organization of LANs. Ultra wide band (UWB) communication is a wireless communication scheme available for such wireless LANs.
The UWB communication is basically designed to perform the baseband transmission by using a pulse train signal having a very narrow pulse width (e.g., 1 ns or less). It has such an occupied bandwidth of the GHz order that the bandwidth divided by the center frequency (e.g., 1 to 10 GHz) becomes about 1. This bandwidth is extremely wide as compared with the bandwidths used for wireless LANs based on the W-CDMA scheme, CDMA 2000 scheme, SS (spread spectrum) scheme, and OFDM (orthogonal frequency division multiplexing) scheme.
The UWB transmission scheme is less interfering to other wireless communication systems due to its low signal power density, and it is expected to be a technique of overlay on frequency bands which are used by existing wireless communication systems. Moreover, due to its wide band, the UWB transmission scheme is expected to be a technique for the super high-speed wireless transmission of the order of 100 Mbps in the field of personal area network (PAN).
The conventional spread-code based communication system has its information signal bit span fixed depending on the length of spread code, and it is problematic in the difficulty of changing the transmission volume. It can solely perform communication at a fixed transmission rate due to the use of UWB signals which are rendered the spreading process based on a spread code formed in a certain bit span.
Due to the communication ability only at a fixed transmission rate, the conventional communication system involves the following problems.
Primarily, a constant high-speed spread code must be used for communication of any distance and any intended speed, and the efficiency is low.
Among communication of different distances, short-distance communication performs normally, whereas long-distance communication becomes difficult due to the interference by short-distance communication.
In short-distance UWB communication, the integration operation continues needlessly even after the threshold value is reached within the length of spread code. In long-distance communication, the threshold value is not reached in the integration operation for a full length of spread code, and long-distance communication cannot perform.